1. Field of the Invention
This invention relates to a wear-resistant protective film for a thermal head and a method of producing a wear-resistant protective film for a thermal head.
2. Prior Art
Thermal heads are extensively used as printing heads for computers, word processors, facsimile machines, etc. The head has a number of dots or resistance heating elements of polysilicon or the like arranged in a matrix and which are selectively supplied with a current to print characters by heat transfer through a printing ribbon onto paper. Since the paper is moved in sliding contact with the thermal head surface, the resistance heating elements must be protected on the surface with a highly wear-resistant protective film.
Each spotlike printing element of the thermal head, as shown in FIG. 1, comprises, from the base upward, a substrate 1 of alumina or the like, a regenerative layer 2 of glaze glass or the like, a heating-element layer 3 of polysilicon or the like, electrodes 4, 5, and a wear-resistant protective film 6. In the figure the numeral 7 designates a heat-developing zone.
The protective film 6 generally is required to have high hardness, limited internal stresses attributable to heat, composition and structure, resistance to wear, and stability to moisture, alkalis, acids and the like. Various materials have hitherto been studied, including such known materials of Si--O--N, Si--Ti--O--N, Si--La--O--N, Si--Al--O--N systems.
Wear-resistant protective films conventionally formed by sputtering crack frequently. Once cracked, such a film allows moisture in the atmosphere to gain entrance through the crack into the thermal head to corrode it, often leading to film separation. Among the factors responsible for the cracking are the development by dint of a peening effect of the internal stresses due to heat, composition, and structure, and the lack of toughness. A particularly serious factor is inadequate step coverage of steplike portions. Ideally, the wear-resistant protective film is formed as shown in FIG. 1. In the actual film-forming process the film material fails to cover the steps fully, as at 8, 8 in FIG. 2, giving cause for cracking as early as the formation of the film. Intrusion of water or repeated exposure to heat would invite premature cracking at the steps.
This step coverage problem can be overcome by the use of a biased radio frequency (RF) sputtering technique in forming a wear-resistant protective film (Japanese Patent Application Public Disclosure No. 135261/1988). The biased RF sputtering proves excellent in coveting steps, but the attendant peening effect and incorporation of sputter gases (Ar, Kr, etc.) into the protective film increase the internal stresses. Consequently, the film cracks easily and becomes less adherent.
Although the above reference describes that cracks and peeling are avoided, the reality is that cracks are prone to develop due to the internal stress, according to the inventors tests. Moreover, there is no disclosure in the reference on forming two or more layers while varying the bias for sputtering.